Laser processed back contact heterojunction solar cells

ABSTRACT

An interdigitated solar cell may provide a heterojunction or tunnel junction emitter and base contacts that comprise laser processed regions that electrically couple the base contact to a substrate. Methods for manufacturing such solar cells to provide interdigitated back contacts may utilize laser processing to form laser processed regions that are isolated from the emitter. Laser processing may include laser-doping, laser-firing, laser-transfer, laser-transfer doping, laser contacting, and/or gas immersion laser doping.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/132,881 filed on Mar. 13, 2015, which is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to interdigitated back contact (IBC) solar cells.More particularly, to systems and methods for fabricating IBC solarcells with a heterojunction or tunnel junction emitter.

BACKGROUND OF THE INVENTION

A desirable solar cell geometry referred to as an interdigitated backcontact (IBC) cell comprises a semiconductor wafer and alternating lines(interdigitated stripes) coinciding with regions with p-type and n-typedoping. This cell geometry has the advantage of eliminating shadinglosses altogether by putting both contacts on the rear side of the waferthat is not illuminated. Further, contacts are easier to interconnectwith both contacts on the rear surface.

Another desirable solar cell architecture involves the use of siliconheterojunction or tunnel junction contacts. A well-known example of sucharchitectures is the HIT (heterojunction with intrinsic thin layer) cellstructure. In the conventional front emitter form of this structure, asilicon wafer is contacted on both sides by a thin intrinsichydrogenated amorphous silicon (a-Si:H) layer, which serves as a surfacepassivating layer as well as a charge carrier transport layer. On thefront of the cell, a semiconductor layer doped to the opposite dopingpolarity of the base substrate is applied, forming a heterojunctionemitter. On the rear of the cell, a semiconductor layer doped to thesame doping polarity as the base substrate is applied, forming a basecontact. These layers can then be contacted with transparent or metallicconducting layers to extract current from the solar cell. In the tunneljunction cell, the intrinsic a-Si:H layer is replaced with a thin highbandgap material. In the case of the heterojunction cell, charge carriertransport occurs via a band conduction mechanism in the intrinsic a-Si:Hlayer, while in the case of the tunnel junction cell, charge carriertransport occurs via quantum mechanical tunneling. Despite thisdifference, these cells operate via similar mechanisms and importantlycan be manufactured in low temperature processes because they do notrequire dopant diffusion.

The conventional heterojunction or tunnel junction solar cells cannotachieve outstanding efficiencies because they still require front sidecontacts. The presence of a contact on the front side firstly reducesefficiency due to blocking or shading of the incoming light by thenecessary metal grids which extract the generated current. Additionally,the presence of a front electrical contact requires that the front ofthe cell be simultaneously optimized for electrical, light absorption,and passivation properties, often producing a compromise which affectscell performance.

Presently, silicon solar cells with the highest efficiency are thosebased on combining an interdigitated all back contact structure withsilicon heterojunction contacts. Panasonic recently reported obtaining arecord conversion efficiency of 25.6% with such a device structure(Masuko et al., 40^(th) IEEE Photovoltaic Specialists Conference, Jun.8-13, 2014, Denver, Colo.). At the same conference, Sharp reportedobtaining an efficiency of 25.1% with a similar device structure(Nakamura et al., 40^(th) IEEE Photovoltaic Specialists Conference, Jun.8-13, 2014, Denver, Colo.), and SunPower obtained an efficiency of 25.0%with an interdigitated back contact (IBC) silicon solar cell made usingconventional diffusion processes (Smith et al., 40^(th) IEEEPhotovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.).While the processing of these high efficiency IBC solar cells were notdiscussed in any detail, the manufacturing costs are likely to berelatively high since the known processing techniques that could beapplied in each case appears to be somewhat complicated with variousmasking and vacuum processing steps required.

While it is clear that back contact heterojunction emitter solar cellscan produce the highest efficiencies, there is a need for improvedmethods for producing these cells in a manner that eliminates theexpense associated with multiple process and alignment steps.Furthermore, there is a need to produce heterojunction or tunneljunction emitter back contact cells with low contact resistance.

SUMMARY OF INVENTION

In one embodiment, a heterojunction emitter back contact solar cell isformed with the emitter comprising a heterojunction and the base contactcomprising a laser processed contact. In some embodiments, the laserprocessed contact may be laser-fired or laser-doped. In someembodiments, the laser processed contact may be formed utilizing lasertransfer doping. In some embodiments, the laser processed contact may beformed utilizing gas immersion laser doping (GILD).

In yet another embodiment, a metal deposition step used to contact theemitter is combined with the metal deposition step used for the laserprocessed contact. In another embodiment, a metal used to contact theemitter and laser processed contact regions is patterned by a laserprocess, such as laser transfer of the metal in a pattern desired orlaser ablation to remove undesired regions of the metal. In anotherembodiment, the emitter occupies 60% or more of the rear surface area ofthe cell.

In another embodiment, a back contact solar cell is formed with theemitter including a heterojunction or a tunnel junction, and theinterdigitated metal fingers are formed by a laser transfer process.Subsequently, the base contact is formed by laser firing or dopingthrough the base interdigitated finger.

In another embodiment, a back contact solar cell is formed with theemitter including a heterojunction or a tunnel junction, and the basecontact is formed using laser firing or doping conditions that disruptthe heterojunction or tunnel junction in the vicinity of thelaser-processed base contact.

In another embodiment, the laser processed contact is formed by a lasertransfer doping process that simultaneously laser fires or laser dopesthe contact.

In some embodiments, either a narrow line-shaped laser beam or a smallGaussian laser beam may be utilized, either of which can be temporallyshaped, to either ablate, transfer a dopant, metal or other material; orto laser-dope or laser fire localized p+ or n+ contacts. The laser beammay also be utilized to deposit or pattern a conductive layer.

In yet another embodiment, a solar cell with interdigitated backcontacts may be provided with base contacts and an emitter. The rearsurface of the solar cell may provide a heterojunction emitter or tunneljunction emitter separated from base contacts by an isolation gap. Thebase contacts may provide laser processed regions where dopant or metalmaterials provide electrical coupling to the substrate. The laserprocessing results in the laser processed regions being isolated fromlayers of the heterojunction emitter or the tunnel junction by resultingablation or the isolation gap. In some embodiments, the isolation gapmay optionally extend through a portion or all of the dopedsemiconductor layer, heterojunction layer, and/or tunnel junction layer.Further, the isolation gap may optionally extend into a portion of thesubstrate. In some embodiments, a passivation or insulating layer may bepresent in between the base contact regions and the emitter region. Insome embodiments, a dielectric layer may be deposited on the basecontact regions. One or more metal layers may be applied to the basecontact regions and emitter regions.

The foregoing has outlined rather broadly various features of thepresent disclosure in order that the detailed description that followsmay be better understood. Additional features and advantages of thedisclosure will be described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1 is an illustrative process flow for a solar cell;

FIG. 2 is an illustrative side view of a cell structure with metalcontact isolation;

FIG. 3 is an illustrative side view of a cell structure with metalcontact isolation and additional patterned metal contacts;

FIG. 4 is an illustrative side view of a cell structure with basecontact laser firing through a dielectric layer;

FIG. 5 is an illustrative side view of a cell structure with metalcontact and doped semiconductor layer isolation;

FIG. 6 is an illustrative side view of a cell structure with completeisolation of deposited layers;

FIG. 7 is an illustrative side view of a cell structure with thesemiconductor doped layer removed in the laser fired contact area;

FIG. 8 is an illustrative bottom view of a cell structure;

FIGS. 9a through 9d are illustrative bottom views of laser firingpatterns; and

FIG. 10 shows an embodiment of a laser-transfer system.

DETAILED DESCRIPTION

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing particularimplementations of the disclosure and are not intended to be limitingthereto. While most of the terms used herein will be recognizable tothose of ordinary skill in the art, it should be understood that whennot explicitly defined, terms should be interpreted as adopting ameaning presently accepted by those of ordinary skill in the art.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only,and are not restrictive of the invention, as claimed. In thisapplication, the use of the singular includes the plural, the word “a”or “an” means “at least one”, and the use of “or” means “and/or”, unlessspecifically stated otherwise. Furthermore, the use of the term“including”, as well as other forms, such as “includes” and “included”,is not limiting. Also, terms such as “element” or “component” encompassboth elements or components comprising one unit and elements orcomponents that comprise more than one unit unless specifically statedotherwise.

FIG. 1 is an illustrative embodiment of a flow diagram for fabricatinglaser contacted heterjunction or tunnel junction solar cell. A suitablesubstrate (step 100) is selected and introduced into the process. Thesuitable substrate may be a semiconductor wafer of any convenient sizeor shape. Nonlimiting examples of suitable semiconductors includes groupIV semiconductors, such as silicon or germanium; group III-Vsemiconductors, such as gallium arsenide or indium phosphide; and groupII-VI semiconductors, such as cadmium telluride. In some embodiments,the substrate thickness is preferably equal to or below about 1 mm. Thesurface of the semiconductor wafer may be polished. In some embodimentsfor solar cell applications, it may be preferable for the starting waferto have a surface that is textured to promote light absorption (optionalstep 110). The surface texture may be applied by mechanical means, laserprocesses, chemical etching processes, or the like. In some embodimentsfor silicon wafers, the preferred surface texture may contain exposureof predominantly <111> and <110> facets, such as is obtained bytreatment with basic solutions containing KOH or NaOH in conjunctionwith surfactants such as alcohols. In some embodiments, the surface maybe chemically smoothed by treatment with various etchants. A startingwafer that has a rough texture resulting from the wafer sawing processcan be chemically smoothed with hydroxide containing etchants, such asNaOH, KOH, TMAH (tetramethylammonium hydroxide), combinations thereof,or the like. The concentrations of these etchants can be greater than10%, and etching may be performed at temperatures equal to or greaterthan 50° C. The resulting surface is substantially smoothed relative tothe starting rough surface, but may still contain pits, depressions, orsurface undulations.

As a nonlimiting example, the surface of the wafer may be textured withvery fine features to produce a gradient refractive index, also referredto as a nanoscale texture or black silicon. In some embodiments, theanti-reflective etching process may be a single stage process thatincludes a catalytic metal and etching chemistries. In some embodiments,the anti-reflective etching process may be a multi-stage process thatincludes: a catalytic metal deposition stage to deposit a metal on thesubstrate, and an etching stage that texturizes surface(s) of thesubstrate to reduce reflectivity. In some embodiments, the catalyticmetal deposition stage may occur utilizing a thin layer fluid processthat includes steps similar to the etching stage as discussed furtherherein. In some embodiments, the catalytic metal may exist in adeposition fluid as a precursor that is reduced or plated on thesubstrate surface. As a nonlimiting example, preferred catalytic metalsolutions contain a catalytic metal and a fluorine containing compound,such as hydrofluoric acid, that is dispensed and/or dispersed into athin fluid layer on the substrate to deposit the catalytic metal on thesubstrate. In some embodiments, the deposition fluid is dispersed orspread out into the thin layer with a thickness of 5 mm or less. Inother embodiments, the deposition fluid is dispersed or spread out intothe thin layer with a thickness of 1.5 mm or less. A thickness of thethin layer of deposition fluid is controlled by controlling a separationdistance between the first surface of the substrate and an opposingsurface of a dispersion mechanism opposite the first surface. Thedeposition fluid may remain in contact with the substrate for about 5seconds to 5 minutes for the catalytic metal deposition stage. After thecatalytic metal deposition stage, the metal catalyst has been depositedon portions of the substrate, and the anti-reflective etching processmay proceed to the etching stage to texturize the surface of thesubstrate where the metal catalyst was deposited on the substrate.

The substrate may have different surface textures on the front and rearsurfaces. The front surface may be textured to promote light absorption,while the rear surface may be textured or smoothed to promotecompatibility with the contacting and laser firing processes. The frontsurface may have textures as discussed above while the back surface maybe a nominally smooth surface obtained by mechanical polishing, chemicalmechanical polishing (CMP), or chemical etching. Differential textureson front and back may be achieved by any conventional means. Thedifferent surfaces may be subjected to different treatments byprotecting one of the surfaces with a protective coating while immersingthe substrate in a treatment bath or processing in a chamber. Thesubstrate may be subjected to a single side process by maintaining thesubstrate partially immersed in a fluid, where one face is immersed andthe other face is not. Alternatively, the substrate may be processed ina chamber where only one side of the substrate is treated.

The starting substrate may be highly pure and thus nearly intrinsic indoping character or may have a particular bulk doping leading it to ben-type or p-type. The presence of doping modifies the bulk resistivityof the substrate. In some embodiments, substrates have a bulkresistivity equal to or between about 0.1 to 50 ohm-cm. In someembodiments, substrates have a bulk resistivity equal to or betweenabout 1 to 25 ohm-cm. The substrate may be an n-type doped silicon grownby the Czochralski method. The examples discussed above and herein areprovided for illustrative purposes only, and it will be recognized thata suitable substrate is in no way limited to the particular examplesdiscussed.

Front passivation layers may be applied (step 120) by any conventionalmeans. As a nonlimiting example, the front passivation may include aprocess such as atomic layer deposition (ALD). The material deposited byALD may include aluminum oxide (Al₂O₃) or silicon dioxide (SiO₂). Apreferred passivation is the ALD deposition of aluminum oxide usingtrimethylaluminum (TMA) as a precursor. The substrate may be annealedafter deposition of the ALD deposited layer to improve or alter thepassivation quality. The front passivation may also be applied byexposing the substrate to oxygen at elevated temperature to produce athermal oxide.

In some embodiments, the front passivation may be a semiconductor layer.A preferred semiconductor passivation layer is hydrogenated amorphoussilicon (a-Si:H). The a-Si:H may be deposited by any conventional means,including plasma enhanced chemical vapor deposition (PECVD) or hot wirechemical vapor deposition (HWCVD). The deposition preferably takes placeat temperatures ranging from equal to or between approximately 150° C.to 450° C., more preferably from equal to or between approximately 200°C. to 400° C. The a-Si:H passivation may be undoped, indicating that nointentional doping compounds are included. Alternatively, the a-Si:Hpassivation layer may be lightly doped by a doping compound. In someembodiments, the a-Si:H layer is relatively thin (equal to or betweenabout 2 to 20 nm) to minimize light absorption at the front surface. Thefront passivation may include several layers of a-Si:H or othersemiconductor materials with various doping levels. In some embodiments,a structure contains a first layer of intrinsic a-Si:H in contact withthe silicon substrate and a second layer of doped semiconductor such asdoped a-Si:H. These and other structures produce a front surface fieldto prevent charge carrier recombination at the front surface.

A rear heterojunction layer or tunnel junction layer (step 130) may beapplied or formed by any conventional means on a rear surface of asubstrate. A heterojunction is the interface that occurs between twolayers or regions of dissimilar crystalline semiconductors. Thesesemiconducting materials have unequal band gaps, as opposed to ahomojunction with materials having equal band gaps. The heterojunctionlayer applied or formed in step 130 may comprise any suitablesemiconductor with the appropriate band structure, mechanical and/oradhesion properties. In some embodiments, the heterojunction material isa-Si:H. The a-Si:H layer can be of any suitable thickness to promotegood charge carrier transport while maintaining integrity. The thicknesscan be in the range equal to or between approximately 2 to 20 nm,preferably equal to or between approximately 4 to 10 nm.

A tunnel junction is a barrier, such as a thin insulating layer or largeband-gap layer, between two electrically conducting materials. Electrons(or quasiparticles) pass through the barrier by the process of quantumtunneling. In other embodiments, a tunnel junction layer is applied orformed in steps 130. A tunnel junction layer may comprise any suitableinsulator or large band-gap material with the appropriate bandstructure, mechanical, and/or adhesion properties. Nonlimiting examplesof tunnel junction materials are SiO₂, SiN_(x), or Al₂O₃. A tunneljunction may be applied by ALD, PECVD, thermal oxidation (whereapplicable), or chemical oxidation (where applicable). In someembodiments, the tunnel junction layer may be formed by a thermaltreatment or chemical reaction of the substrate. As a nonlimitingexample, a silicon substrate may be subjected to thermal or chemicaloxidation to form a SiO₂ layer or tunnel junction layer.

A doped semiconductor layer (step 140) may be applied by anyconventional means on a rear surface of the substrate below theheterojunction layer or the tunnel junction layer. A rear dopedsemiconductor layer may comprise any suitable semiconductor with theappropriate band structure, mechanical and/or adhesion properties.Preferred nonlimiting example of a doped semiconductor layer is dopedsilicon. The silicon may be amorphous, microcrystalline, orpolycrystalline depending upon growth conditions. The silicon layer maybe deposited by any conventional means, including plasma enhancedchemical vapor deposition (PECVD) or hot wire chemical vapor deposition(HWCVD). The deposition preferably takes place at temperatures rangingequal to or between approximately 150° C. to 450° C., and morepreferably equal to or between approximately 200° C. to 400° C. Dopingmay be accomplished by including a chemical dopant in the depositionfeed stream. Nonlimiting examples of dopant chemicals include borane,diborane, phosphine, and arsine. The dopant may be present in the feedstream at concentrations from equal to or between approximately 0.5% to20% on a molar basis relative to the silicon precursor. Preferredconcentrations are from equal to or between approximately 2% to 10%. Insome embodiments, the step 140 of applying doped semiconductor layer mayinclude patterning to provide gaps between emitter regions and baseregions or to remove the doped semiconductor layer from the baseregions. In some embodiments, the patterning to provide gaps discussedabove may also remove a portion of the heterojunction layer or thetunnel junction layer. The patterning may involve masking, etching, orthe like form a desired pattern for the doped semiconductor layer.

In embodiments providing a heterojunction, the heterojunction layer andthe rear doped semiconductor may be referred to collectively as aheterojunction structure since the layers function together to producethe proper energy band characteristics in their vicinity. It shall beunderstood by one of skill in the art that any heterojunction structurediscussed herein refers to any single or combination of layers employedto provide a heterojunction that provides the desired energy bandcharacteristics. In embodiments providing a tunnel junction, the tunnellayer and the rear doped semiconductor may be referred to collectivelyas a tunnel junction structure since the layers function in order toproduce the proper energy band characteristics in their vicinity. Itshall be understood by one of skill in the art that any tunnel junctionstructure discussed herein refers to any single or combination of layersemployed to provide a tunnel junction that provides the desired energyband characteristics.

In some embodiments, a dielectric layer (step 145) may be applied and/orpatterned on the doped semiconductor layer. The dielectric layer may bepositioned on the doped semiconductor layer in the base contact regions.The dielectric layer may be discontinuous and provide an opening or gapin the emitter region. In some embodiments, the dielectric layer may belarger than the base contact regions so that a portion of the dielectriclayer extends into the region between the base contact region and/orinto a portion of the emitter region. The dielectric layer may providefunction(s), such as passivation or protection of the underlyingregions.

Laser processing of the base contact (step 150) may be performed by oneor more processes including laser firing, laser doping, laser transfer,laser transfer doping, laser-contacting, laser ablating of materials,and/or gas immersion laser doping (GILD). Laser processing generallyrefers to any laser processing step(s) that are utilized to produce thebase contacts, and it shall be understood that laser processing mayinclude one or a combination of steps, such as the steps listed above.Laser-firing or laser-doping may be utilized herein to refer to anylaser processing where the laser beam locally disrupts or removesmaterials (such as a doped semiconductor layer, heterojunction/tunneljunction layer, and a portion of the substrate), and simultaneouslydrives a desired material (such as a dopant or metal dopant) into thesubstrate. Laser-transfer may be utilized herein to refer to laserprocessing where the laser beam transfers material(s) from a donorsubstrate onto a desired substrate. In some embodiments, laser-transfermay be utilized outside of the laser processing step 150 to depositdesired materials, such as metal contact layers. Laser-transfer dopingmay refer to utilizing the laser-transfer discussed above combined withlaser-firing or laser-doping. It should be noted that the laser-dopingor laser-firing may occur simultaneously with the laser-transfer, oralternatively, may be performed as distinct steps. In some embodiments,laser ablating may be a by-product of the laser processing. For example,setting of a laser can be tuned so that the laser causes thelaser-processed regions to be isolated from nearby layers (e.g. dopedsemiconductor layer), such as by creating a small gap in between.Laser-contacting may be utilized herein to refer to any process in whichthe laser is used to at least partially produce a contact for thesubstrate. The process may include a single above described function,such as laser transfer or laser doping. Alternatively, the process mayinclude multiple functions that occur simultaneously, serially, orthrough several laser treatments from one or more laser systems. Suchmultiple function laser contact may include a laser transfer dopingprocess in which the laser pulse transfers materials and dopes thesubstrate, yielding a useful contact on the substrate.

The laser processing step 150 is performed on base contact regions toform laser-processed regions that extend through any layers present onthe rear surface of the substrate (e.g. the doped semiconductor layer,intrinsic layer or insulating/large band-gap layer, and/or dielectriclayer) to the substrate. This allows subsequently deposited metal layersin the base contact regions to be electrically coupled to the substrate.Further, the laser processing step 150 may also result in localizedablating of any layer(s) present to minimize or eliminate electricalcontact with the laser processed regions, which improves isolation ofthe base contacts from the emitter.

In some embodiments of a laser processing step, a dopant source may beoptionally applied to the silicon substrate and driven into the siliconduring the laser pulse, which may be referred to herein as laser-firingor laser-doping. In some embodiments, the laser processing step 150comprises laser-firing or laser-doping where the laser beam causeslocalized disruption of the materials on a substrate, and a dopant ormetal dopant from dopant material(s) applied to the substrate is driveninto the substrate during the laser pulse. As a result, a laserprocessed region is formed with the dopant or metal dopant that provideselectrical contact with the substrate, and any doped semiconductor,heterojunction, tunnel junction, and/or other layers are removed orisolated from the laser processed regions. In some embodiments, thedopant layer may be applied by physical vapor deposition techniques,including evaporation and sputtering. In some embodiments, the dopantlayer may be applied by chemical vapor deposition techniques. In someembodiments, the dopant layer may also be applied by liquid depositiontechniques, including screen printing, spin coating, bead coating, orinkjet printing. The liquid dopant can be supplied at a solution ordispersion or slurry.

In some embodiments of the laser processing step, the dopant may besupplied from an donor substrate, which may be referred to herein aslaser-transfer. FIG. 10 shows a nonlimiting embodiment of alaser-transfer system. Substrate (200), anti-reflection layer (210),heterojunction/tunnel junction layer (220), and doped semiconductorlayer (230) may correspond to layers discussed in other embodimentsdiscussed further below. A coating (400) is present on the surface of adonor substrate (410). The donor substrate is maintained at a distanceor gap (420) from the substrate (200), which is the substrate that willreceive the material(s) to laser-transferred. The distance or gap (420)between the donor substrate (410) and the nearest layer (e.g. dopedsemiconductor 230) of substrate (200) may range from being in directcontact with each other to 0.5 mm. A laser beam (430) directed throughthe donor substrate (410) toward the main substrate (200) has the effectof vaporizing the coating (400) in a region (440) exposed to the beamand transporting the desired material to coat the main substrate. Insome embodiments, the power from the laser pulse (430), or additionalpulses, may serve to incorporate the desired material or a derivativethereof into the main substrate (200). The coating (400) may be a dopantmaterial or a derivative thereof, so that the above laser-transferprocess results in a doped region (250) in a base region (252) separatedfrom the emitter (255). In some embodiments, the coating (400) may be asemiconductor, conductor or precursor thereof. In such cases, the lasertransfer process may produce regions or desired properties on thesubstrate (200), such as regions of high conductivity. In contrast tolaser-firing where dopant or metal dopant materials are present on thesubstrate, laser-transfer causes a desired material(s) from a donorsubstrate (410) to migrate onto a donee or main substrate (200).Laser-transfer can be taken even further to cause doping of the mainsubstrate as well, which may be referred to herein as laser-transferdoping, such as where the dopant is driven into a certain depth of thedonee substrate. In some embodiments, the laser-transferring ofmaterials and the doping may occur simultaneously. This ‘doping’associated with the laser-transfer process is akin to the laser-dopingor laser-firing discussed above (except the dopant being supplied from adonor substrate), but is referred in this section discussinglaser-transfer as ‘doping’ for the sake of clarity. In otherembodiments, the laser-transferring and doping may be separate,individual processing steps. The laser-transfer approach utilizes arapid interaction between a laser and a non-transparent thin source filmdeposited on a transparent plate or donor substrate (e.g. glass orquartz), which is placed in close proximity (e.g. about several microns)to a receiving substrate. The source materials supplying the dopant ormetal dopant may be applied to the donor substrate by any suitableapproach or any of the approaches mentioned above. The donor substratemay also include thin flexible glass and/or polymer films and othermaterials transparent to the laser radiation.

The donor or transfer substrate of the laser processing system can becoated with multiple layers depending on the application. As anonlimiting example, the laser transfer substrate may be first coatedwith a thin easily evaporated material (e.g. a-Si:H) to act as a releaselayer for a refractory material (e.g. Mo) or a transparent material(e.g. SiO₂) deposited on the a-Si:H. Another example involves firstdepositing a layer of Ni on the laser transfer substrate followed by alayer of Sb so that the laser will transfer Sb for n⁺ doping and Ni fora low-resistance nickel silicide contact.

The laser processing system can utilize multiple pulses in addition totemporally shaped pulses. In some embodiments, a laser beam may betemporally shaped with an initial energy density over a first timeperiod to efficiency transfer the dopant material from a donor plate tothe substrate. Further, the laser beam may also be tuned to disrupt theheterojunction or tunnel junction structure to cause delamination. Infurther embodiments, the laser beam may be transitioned to a secondenergy density that is lower that the initial energy density for asecond time period to locally melt the heterojunction or tunnel junctionstructure and to allow a dopant to diffuse into the substrate. Infurther embodiments, the laser beam may be transitioned to a thirdenergy density that is lower than the second energy density for a thirdtime period to anneal the localized laser-processed region. As anonlimiting example, the first pulse could comprise a first section ofrelatively high energy density (e.g. ˜1 j/cm²) over a predetermined timeperiod (e.g. 10 ns), and then a slowly decreasing section where theenergy density decreases, such as from equal to or between approximately0.7 to 0.1 J/cm² over a predetermined period of time (e.g. 400 ns). Asecond pulse to the same location might then be applied approximately aset period of time (e.g. 10 μs) later (100 kHz repetition rate) with anenergy density ramping up (e.g. ˜0.3 J/cm²) over a fixed period of time(e.g. 10 ns). The second pulse may then slowly decrease (e.g. 0.05J/cm²) over a predetermined period of time (e.g. 500 ns) to furtheranneal the treated region. The wavelength of the laser beam can be inthe IR (e.g. 1064 nm) for most applications, but a laser beam operatingin the green (532 nm) can also be used and will more effectively heatjust the top few μm of an exposed Si surface. The IR beam will initiallyheat the Si wafer to a depth of a few hundred μm, but as the laserrapidly heats up the Si locally, the absorption coefficient in the IRincreases rapidly and the heating becomes localized near the surfaceregion.

In some embodiments, a GILD or Gas Immersion Laser Doping process may beutilized in the laser processing step 150, where the dopant is suppliedin vapor form to a chamber above the substrate such that the dopantvapor is in gaseous communication with the substrate. The gaseous dopantor a byproduct is incorporated into the substrate during the laserpulse. Gaseous source(s) can be any material containing a dopant atomwith sufficient volatility, including, but not limited to, POCl₃, PCl₃,PH₃, BH₃, B₂H₆, arsine, and trimethylaluminum.

The various laser processes discussed above may utilize spatially and/ortemporally shaped laser beams. In some embodiments, the systems ormethods discussed herein may have the following elements: (1) supply ofdopants by a laser process; (2) dopants supplied by the laser process toavoid heating of the wafers to perform dopant diffusion; and/or (3) aninterdigitated back contact (IBC) cell. The use of line beams is aparticularly attractive way to make an IBC cell since the electrodes ofthe IBC are thin lines, and thus can be patterned with single or reducednumber of laser pulse exposures. In some embodiments, the combination of(1)-(3) above may be utilized with line and/or temporal shaping. In someembodiments, the laser beam can be spatially shaped into a narrowline-shaped laser beam or into an array of small diameter Gaussian laserbeams (e.g. ≦20 μm or ≦10 μm). Line-shaped laser beams with widths ≦10μm exhibit little laser-induced damage, while conventional circularGaussian laser beams (e.g. with diameters of ˜30-130 μm) exhibitmicrocracks and dislocations. Small diameter (≦20 μm or ≦10 μm) Gaussianlaser beams are also less likely to exhibit extended defects, such asmicrocracks and dislocations due to the fact that only a very smallregion of Si is melted and recrystallized. In some embodiments, one oran array of Gaussian beams may be utilized, where the Gaussian beams maybe circular and/or each of the Gaussian beams has a diameter of 30microns or less. In some embodiments, a laser beam that is narrow andline-shaped may be utilized. Further, the width of the laser beam may be20 microns or less.

The temporal pulse shape can be selected for the purposes of lasertransfer of material, laser ablation or disruption of dielectricpassivation layers, laser melting of selected localized regions of theSi wafer, laser doping of the melted Si regions with the appropriatedopant atoms, laser firing of contacting metals through the dielectricpassivation layers and/or laser annealing of the localized treatedregions on the Si wafer. Generally, laser transfer of material requiresrelatively short pulses (e.g. few ns to few tens of ns), while laserannealing requires relatively long pulses (e.g. 0.1 μs to several μs).The pulse duration for laser doping will depend on the dopant depthdesired and can vary from tens of ns to hundreds of ns. In someembodiments, the dopant penetration depths are from 0.02 μm to 1 μm,preferably 0.1 μm to 0.5 μm. As a nonlimiting example, a laser processwhich combines laser transfer, disruption of the dielectric passivation,melting, doping and annealing of the Si in a localized region mightemploy a line-shaped beam (e.g. 8 μm wide and 1 cm long) with thefollowing temporally shaping: the pulse starts with an initial energydensity (e.g. ˜1 J/cm²) over several ns to transfer the dopant material(e.g. Al) to the substrate (e.g. Si surface) and disrupt the dielectricpassivation, if present (e.g. 5 nm of ALD Al₂O₃/90 nm of PECVD SiO_(x)on the rear surface); the energy density then falls (e.g. ˜0.5 J/cm²)over a set period of time (e.g. ˜50 ns) to locally melt the substratesurface and diffuse in the dopant. Then the pulse energy densitydecreases (e.g. 0.5 to 0.1 J/cm²) over time (e.g. ˜400 ns) to anneal thelocalized region of substrate surface.

As nonlimiting examples, dopant materials may be any suitable n- orp-type material, Al, Sb, Group III or V element, or the like. In thelaser-transfer process, the dopant atom is introduced on a donorsubstrate containing or coated with a dopant material including thedonor atom. In a laser-firing process, the dopant is included as a metalor dopant material, respectively, on the substrate to be fired. In aGLID process, the dopant is provided in vapor form. The dopant materialmay be a pure form of the dopant, such as but not limited to coatings ofthe group III or group V atoms. Alternatively, the dopant material maybe a compound containing the dopant, such as but not limited to anoxide, nitride, or chalcogenide of the donor. The dopant material mayalso be composed of a host material containing the dopant, such ashydrogenated amorphous silicon heavily doped with the dopant.Concentration of the dopant in the host material may be greater than0.5%, preferably greater than 2%. The spacing of the interdigitatedfingers in the laser-transferred line-contact IBC cells can be maderelatively small (e.g. 100-300 microns), so that the lateral resistance(electrical shading) in the device is small.

In some embodiments, the laser processing step 150 may be utilized toremove materials by causing laser ablation or delamination due tolocalized heating, such as to create an isolation gap or the removeportions of a layer that are not desired in base contact regions. Insome embodiments, processing conditions of a laser doping system duringthe laser processing step 150 may be tuned to for this remove materials.In some embodiments, the laser doping system may be tuned to remove aportion or all of a doped semiconductor layer, intrinsic layer orinsulator/high band-gap layer, and/or substrate in a region between thebase contact and emitter to create an isolation gap. In someembodiments, the laser doping system may be tuned to remove a dopedsemiconductor layer in the base contact regions.

In some embodiments, a passivation layer (step 155) may be optionally beapplied and/or patterned on the doped silicon layer. The passivationlayer may be positioned on the doped silicon layer between the basecontact and emitter regions to provide electrical isolation. While thepassivation layer step 155 is shown before application of the metalcontacting layer in step 160, in other embodiments, the passivation stepmay optionally be performed after the metal contacting layer step 160.

A metal contacting layer (step 160) may be applied by any conventionalmeans. While application of the metal contacting layer (step 160) isshown after performing laser processing of the base contact (step 150),in other embodiments, the application of the metal contacting layer maybe performed before the laser processing step, such as when laser-firingis desired to drive the applied metal into the substrate, or combinedwith the laser processing step. Nonlimiting example of the means ofdeposition may include physical vapor deposition techniques such asvacuum evaporation or sputtering, chemical bath techniques such aselectroplating or electroless plating, and liquid techniques such asscreen printing, bead coating, and inkjet printing. The metal contactinglayer may include any suitable metal with sufficient conductivity andcontact conductance to allow current extraction from the cell.Nonlimiting examples include aluminum, silver, copper, nickel, gold, andantimony. The metals may be present as alloys or two or more metals oras multilayer structures of two or more metals. The alloy or multilayerstructures may be used to promote adhesion, or to simultaneously providea doping source. Nonlimiting examples of multilayer structures mayinclude aluminum and silver, or antimony and silver.

The metal layers may be annealed after deposition to promote variousproperties including increased conductivity or improved contactconductance. Anneals may be performed at temperatures ranging from equalto or between 50° C. to 500° C., preferably equal to or between 100° C.to 400° C. Nonlimiting examples of anneal environments may include air,oxygen, nitrogen, mixtures of an inert gas and hydrogen. The anneal maybe performed at constant temperature or at a variety of temperaturesthrough processes of temperature holds and/or ramps. The temperaturetreatments may occur in rapid thermal processing environments such as arapid thermal processing chamber or a belt furnace. In some embodiments,the metal layer and/or anneal may be performed using laser processing.The laser processing may be applied to a portion of the substrate. Thelaser processing may have variations in focus size, intensity, and/orresidence time at a particular substrate location in order the yielduseful heating and cooling profiles for the anneal.

If necessary, patterning of the metal layers (step 170) can be performedafter application of the metal contacting layer in step 160.

In some embodiments, an interdigitated finger pattern can be formed byusing a line-shaped laser beam to deposit a seed layer in a desiredpattern, which may be plated with a highly conductive metal. In someembodiments, the seed layers could then be plated with a metal, such asNi, Ti, or the like. Further, this may be optionally followed by platingwith a more conductive metal, such as Al, Ag, Cu, or the like, to form ahighly conductive interdigitated finger pattern. Subsequent to the lasertransfer process, the solar cell can be annealed at moderatetemperatures (200-450° C.) to improve the electrical properties of thecontacts by promoting silicide formation and inducing atomic hydrogenmotion from the PECVD SiO_(x):H into the Si to passivate anylaser-induced defects.

In some embodiments, and interdigitated finger pattern can be formed byselectively removing metal from the deposited metal layer. The depositedlayer may be in contact with both the heterojunction emitter and thelaser processed base contacts. Patterning may be performed by using apatterned resist to cover or protect portions of the metal layer whileexposing the metal layer to an etching environment which attacks areasthat are not protected. The resist may be a photoresist which ispatterned by exposure to radiation. The resist may be a laser ablatableresist, which can be patterned by direct action of a laser beam or otherhigh intensity light source. Laser ablatable resists include polymerswith the necessary decomposition properties. The metal layer may also bepatterned by direct ablation of the metal using a laser beam or otherhigh intensity light source.

In some embodiments, an insulating layer (step 175) may be appliedand/or patterned into gaps in the metal contacting layer. The insulatinglayer may be present in the gaps in the metal contacting layer and ontop of the metal contacting layer. The portion of the insulating layeron metal contacting layer in the emitter region may allow subsequentmetal deposited for the base contact to extend over the emitter regionwithout making electrical contact with the emitter. This allows anysubsequent additional metal layers (e.g. 270, FIG. 3) for the base andemitter to provide approximately the same width at the bottom most orexposed portion of the additional metal layers.

Additional metal layers may be applied (step 180) for functions, suchas, but not limited to improving the conductance of the metal contactand protecting the metal contact from the environment. Nonlimtingpreferred methods of applying the additional metals includeelectroplating, electroless plating, and screen printing.

After step 180 the cell may receive additional processing such asannealing, encapsulation, and other protective or passivation measures.

The substrate is optionally cleaned at any point in the process asdesired or required. Cleaning of the substrate may be done by solutionmeans, including, but not limited to, treatments with acids, bases, andoxidizing chemistries. Nonlimiting examples of suitable cleaningsolutions include the RCA process, involving exposure to at least (1) asolution including HCl and H₂O₂; (2) a solution including NH₄OH andH₂O₂; and (3) a solution including HF. The exposure to cleaningsolutions can include exposure to any combination of them in anysuitable order. Cleaning can also include other solution exposures, suchas the Piranha etch, comprising H₂SO₄ and H₂O₂, or solvent exposures, orcleaning in water. Useful solvents include alcohols, ketones,hydrocarbons, or halogenated solvents. Cleaning can also involve dryprocesses. These include ozone exposures, corona discharge treatments,plasma treatments, or the like. The treatments may be intended to cleanthe surface; however, it may be useful to combine cleaning withtreatments that etch the surface.

It should be recognized that the order of the steps shown in FIG. 1 maybe modified as necessary in other embodiments to achieve high performingcells, such as to provide various illustrative embodiments discussedfurther herein. It shall be understood that the following illustrativeembodiments of solar cells with IBCs discussed below with reference tothe figures are nonlimiting examples of solar cells that may befabricated utilizing the process discussed with reference to FIG. 1.

FIG. 2 is an illustrative embodiment showing a side view of a solar cellwith an IBC, which is formed by the fabrication process discussed above.A substrate (200) has a passivation and antireflection structure (210)on its front surface. In some embodiments, the back surface contains aheterojunction including an intrinsic or heterojunction layer (220) onthe rear surface of the substrate and a doped semiconductor layer (230)below. The intrinsic layer (220) may be any suitable semiconductor, suchas a-Si:H. The intrinsic or heterojunction layer (220) and dopedsemiconductor layer (230) collectively may be referred to as aheterojunction structure. In other embodiments where a tunnel junctionis desired, a tunnel junction layer (220) and doped semiconductor layer(230) below may be substituted on the rear surface of the substrate, andthe tunnel junction layer may be an insulator layer of insulatingmaterial or large band-gap layer of a material with a large band-gap.Nonlimiting examples of suitable tunnel junction materials may includeSiO₂, SiN_(x), or Al₂O₃. In the case of a tunnel junction, tunneljunction layer (220) and doped semiconductor layer (230) collectivelymay be referred to as a tunnel junction structure.

After the heterojunction (or tunnel junction) layer (220) and dopedsemiconductor layer (230) are formed, laser processed regions (250) arefabricated to penetrate through the heterojunction layer (220) and dopedsemiconductor layer (230) to form part of a base contact (252). Thelaser processing may cause the laser processed regions (250) to beelectrically isolated from the heterojunction/tunnel junction layer(220) and doped semiconductor layer (230). Electrical isolation of thelaser processed regions (250) may also occur naturally from theproperties of the adjacent layers. As a non-limiting example, the dopedsemiconductor layer (230) may be constructed with sufficiently lowlateral conductivity that the laser processed region (250) iseffectively isolated from the majority of the heterojunction or tunneljunction structures. The laser-processed doped regions (250) may also betuned for a minimum penetration into the silicon substrate 200. Thisminimum penetration may include a penetration through the dopedsemiconductor layer (230) and just beyond the intrinsic layer (220). Ametal layer (240) is applied and/or patterned on the doped semiconductorlayer (230) to provide regions in electrical communication with the basecontact (252) and regions in contact with heterojunction/tunnel junctionlayer (220) and doped semiconductor layer (230) that form the actualemitter (255). The metal layer is separated between base contact (252)regions and emitter (255) regions by isolation gaps or openings (260)that provides isolation between the base and emitter contact structures.In the embodiment shown, the isolation gap extends to, but not through,the doped semiconductor (230) region. As utilized herein, discussion ofthe electrical ‘isolation’, or simply isolation, of the base and emittershall be understood to indicate that electrical current flow betweenbase and emitter is kept within a range of values that do notsubstantially harm the device performance. In some embodiments, thiselectrical current flow may be associated with the shunt current.Isolation or electrical isolation may indicate that the shunt currentbetween the base and emitter is equal to or less than 20 mA/cm² duringcell operation, or more preferably equal to or less than 10 mA/cm².Alternatively, in other embodiments, the isolation or electricalisolation may be characterized by the resistance between the base andemitter. For example, isolation or electrical isolation may be indicatedby a resistance between the base and emitter that is equal to or greaterthan 20 ohm-cm², or more preferably equal to or greater than 100ohm-cm². Without being bound by theory, it is believed that the laserprocessing may result in an advantageous disruption of layers comprisingthe heterojunction (or tunnel junction) structure, thus aiding inproducing good isolation. An optional extra metal layer (270) positionedon metal layer (240) is illustrated, which like the metal layer providesgaps or openings in between base contacts (252) and the emitter (255).In some embodiments, the doped semiconductor layer may be treated inregions such as the isolation gap (260) to modify its electricalproperties to a reduced conductivity or nonconductive state. Thetreatments may include locally heating with a laser or other highintensity light sources, chemical, or plasma treatments. A non-limitingexample is treatment of the exposed area (260) of the semiconductorlayer with a laser beam and/or chemical environment in order to reducethe conductivity of the layer in that exposed region. This approach canimprove the isolation between base and emitter contacts. In someembodiments, an optional passivation or protection layer (280) may bepresent on the doped semiconductor layer (230) between the base contact(252) regions and emitter (255) regions. The passivation or protectionlayer can be deposited prior to the metal deposition (as shown) or afterthe metal deposition.

FIG. 3 is an illustrative embodiment showing a side view of anotherembodiment of a solar cell with an IBC formed by the fabrication processdiscussed previously. The structure, arrangement, and process forproviding the components of the solar cell may be identical or similarto those discussed above for FIG. 2, except as discussed further below.In some embodiments, it may be desirable to maximize the width of theemitter region (255) at its widest point. In order to maintain lowseries resistance, the optional extra metal layer (270) can be patternedwith the aid of an insulating masking layer (290) in order to permit thedimensions of the exposed width of the extra metal layer on both thebase (252) and emitter regions (255) at their respective widest pointsto be similar. Insulating masking layer (290) may be applied andpatterned into the isolation gap (260) and over a portion of the metallayer (240) in the base (252) and emitter regions (255). In oneembodiment, the width of the extra metal layer (270) at its widest pointover the emitter region (255) is within +/−40% of the width of the extrametal layer at its widest point over the base region (252). Theinsulating masking layer (290) provides isolation of the base (252) andemitter regions (255). Insulating masking layer (290) fills theisolation gap (260) and covers a portion of the metal layer (240) in theemitter region (255), thereby allowing the extra metal layer (270) forthe base contact to lie over regions of the emitter contact withoutmaking electrical contact with the emitter. Insulating masking layer(290) may also cover a portion of the metal layer (240) and fill spaceunoccupied by the metal layer on a side of the base contact (252) thatis opposite the emitter region (255). The insulating masking layer canbe any insulating material including inorganic insulators, such as SiO₂,SiN_(x), or Al₂O₃, or organic insulators such as polymers. Theinsulating masking layer can be patterned by printing, materialtransfer, or selective deposition prior to deposition of the extra metallayer (270). The insulating masking layer can be patterned byphotolithography or application of a resist material, and can itself bea photoresist or other light or radiation sensitive material.

FIG. 4 is an illustrative embodiment showing a side view of anotherembodiment of solar cell with an IBC formed by the fabrication processdiscussed previously. The structure, arrangement, and process forproviding the components of the solar cell may be identical or similarto the embodiments discussed above, except as discussed further below.In some embodiments, the base regions (252), particularly on the dopedsemiconductor layer (230), may be covered with a dielectric layer (295)prior to laser processing. In some embodiments, the dielectric layer(295) may cover a portion of the base (252) regions. Alternatively, thedielectric layer (295) may extend past the base regions in one or moredirections and may extend over portions of the emitter region (255).During solar cell operation, portions of the metal (240) that are incontact with the base laser processed regions (250) will develop apotential relative to the doped semiconductor layer (230). Thispotential may allow injection or extraction of charge carriersthroughout the contact regions between base contacted metal (240 incontact with 250) and the doped semiconductor layer (230), resulting ina loss of current. An insulator, such as dielectric layer (295), thatsubstantially occupies the possible contact area will diminish orprevent this loss of current. In the embodiment shown, the laser dopedregions (250) are formed by laser processing step performed after thedeposition of the dielectric layer (295) and extend through thedielectric layer and other layers present on the rear surface to thebase (252) regions of the substrate (200). The portion of metal layer(240) that forms the base (252) contact (e.g. corresponding to (304) inFIG. 9a ) are deposited on the dielectric layer (295) and connects tothe laser doped regions (250) while having less contact or no contactwith the doped semiconductor layer (230).

FIG. 5 is an illustrative embodiment showing a side view of anembodiment of a solar cell with an IBC formed by the fabrication processdiscussed previously. The structure, arrangement, and process forproviding the components of the solar cell may be identical or similarto the embodiments discussed above, except as discussed further below.The device shown is similar to that of FIG. 2, except that the contactisolation between base region (252) and emitter (255) provided byisolation gaps (260) extends through the doped semiconductor layer (230)to provide additional electrical isolation between the emitter and thebase contact structures. In some embodiments, the process to etchsubstantially though the doped semiconductor layer (230), while leavingthe intrinsic layer (220) substantially intact, to form isolation gaps(260) may be performed after deposition of the doped semiconductor layerand can include a wet etch process or a dry etch process. The processcan include a selective etching chemistry that etches the dopedsemiconductor layer at a higher etch rate than the intrinsicsemiconductor layer. The materials selected as the doped semiconductorlayer and intrinsic layer may be chosen to enhance the selective etchingof one layer over the other. In other embodiments, the isolation gaps(260) may be formed during laser processing by ablating the dopedsemiconductor layer (260) where the isolation gaps are desired.

FIG. 6 is an illustrative embodiment showing a side view of anembodiment of a solar cell with an IBC formed by the fabrication processdiscussed previously. The structure, arrangement, and process forproviding the components of the solar cell may be identical or similarto the embodiments discussed above, except as discussed further below.The device shown is similar to the FIGS. 2 and 5, except that theisolation channel provided by isolation gaps (260) extends through allof the doped semiconductor layer (230), heterojunction/tunnel junction(220) layer, and optionally a small depth into the base (265) ofsubstrate (200). In other embodiments, the isolation gap (260) may onlyextend partially through the heterojunction/tunnel junction layer (220).It shall be apparent that FIGS. 2, 5 and 6 show the range of depths ofthe isolation feature (260) that are contemplated.

FIG. 7 is an illustrative embodiment showing a side view of anembodiment of a solar cell with an IBC formed by the fabrication processdiscussed previously. The structure, arrangement, and process forproviding the components of the solar cell may be identical or similarto the embodiments discussed above, except as discussed further below.The device shown is patterned prior to the deposition of the metal tosubstantially remove the semiconductor doped layer (230) in the regionof the base contacts (252), and optionally a portion of thesemiconductor doped layer in surrounding areas may also be removed. Itshall be apparent that laser processed region (250) is laser processedprior to removal of a portion of the semiconductor doped layer (230)since the laser processed region extends from depth corresponding tosemiconductor doped layer. The removal of the semiconductor layer (230)can include partial removal of the layer, full removal of the layer,removal of some or all of the intrinsic layer (220), and/or removal ofsome of the base substrate (200). In some embodiments, removal of thesemiconductor doped layer (230) and optionally a portion of theintrinsic layer (220) (or heterojunction layers) in the regionsurrounding the laser-processed base contact (252) may be achievedduring the laser processing step by choosing laser processing conditionsthat cause both the doped and the intrinsic layers to delaminate due tolocalized heating. In other embodiments, the semiconductor doped layer(230) and the intrinsic layer (220) (or heterojunction layers) in theregion surrounding the laser-processed base contact (252) may be removedby other suitable means, such as etching. In other embodiments, thelaser processing to form laser processed regions (250) may be performedafter removal of the semiconductor doped layer (230).

In some embodiments, a passivated solar cell is provided where most ofthe rear surface contains a tunnel oxide emitter interspersed withparallel lines of ohmic base contacts in a finger pattern. The desiredarrangement may be formed by laser ablating the tunnel oxide layer toisolate the base contact and emitter, and laser transferring a basecontact finger pattern using laser beams. In one embodiment, a tunneloxide layer is first deposited on the rear surface by atomic layerdeposition, and then a thin layer of metal oxide(s) (e.g. MoO_(x) andZnO) are then deposited. A line-shaped laser beam is used to ablate aline region to create a gap region that provides isolation between thebase contact and emitter, and then a line is laser transferred and dopedin the central region (e.g. Sb) of the base contacts to createlaser-processed doped regions that extend through an intrinsic layer fora heterojunction or an insulator/high band-gap layer for a tunneljunction to contact the substrate. In some embodiments, it may bepossible to laser transfer the Sb under conditions that locally disruptthe tunnel oxide layers so that a separate laser ablation step is notrequired; thus, some embodiments may contemplate a combined laserablation and doping step. Further, both the tunnel oxide layers and theSb base contact may be plated with conductive material (e.g. Ni/Cu) toincrease the conductivity of the contacts.

FIG. 8 is an illustrative embodiment showing a bottom view of a solarcell with an IBC formed by the fabrication process discussed previously.It shall be apparent to one of skill in the art that any of the variousembodiments discussed above and illustrated with side views (e.g. FIGS.2-7 & 10) may have the same or a similar interdigitated finger patternas illustrated in FIG. 8. The structure, arrangement, and process forproviding the components of the solar cell may be identical or similarto the embodiments discussed above. The contact metal (240) is portionedinto a base finger (304) and an emitter finger (306), each having apredetermined width determined in the Y direction. In some embodiments,it is preferable that the width of the base finger or finger portion ofthe base contact (304) be less than the width of the emitter finger orfinger portion of the emitter (306). In some embodiments, it ispreferable that the emitter finger (306) comprise greater than 60% ofthe total area of all of the finger structures. An isolation gap (260)separates and isolates base fingers (304) from emitter fingers (306).The isolation gap (260) may be of any suitable size that is easilymanufacturable. In some embodiments, it is preferably that the isolationgap be equal to or less than 200 um wide, preferably equal to or lessthan 100 um wide. The base laser firing region (300) illustrateslaser-processed doped regions that extend through an intrinsic layer fora heterojunction or an insulator/high band-gap layer for a tunneljunction to contact the substrate. The base laser firing region (300)should be contained within the base finger so that contact metal (240)for the base finger (304) is in contact with the base laser firingregion.

FIGS. 9a though 9 d are illustrative embodiments showing a bottom viewof a solar cell with an IBC formed by the fabrication process discussedpreviously and demonstrating different embodiments of base contact laserfiring patterns. It shall be apparent to one of skill in the art thatany of the various embodiments discussed above and illustrated with sideviews (e.g. FIGS. 2-7 & 10) may have the same or a similarinterdigitated finger pattern as illustrated in FIGS. 9a-9d . Thestructure, arrangement, and process for providing the components of thesolar cell may be identical or similar to the embodiments discussedabove. FIG. 9a shows a circular beam point contact structure. FIG. 9bshows a line beam point contact structure. In some embodiments, thelaser firing patterns may not overlap or provide separation betweenadjacent firing points. The separation between points in the circular orline beam contact structures can be equal to or less than 2 mm,preferably equal to or less than 1 mm. FIG. 9c shows a line beam linecontact, in which the line beam features overlap to form a continuousline of laser firing. In some embodiments, the laser firing pattern mayinclude more than one parallel line in the X direction. FIG. 9d shows anarray of line beam point contacts with discrete spacing in both the Yand X directions. In some embodiments, the line beam point contacts maybe substituted with circular beam point contact structures oroverlapping line beam features to form an array. The array is preferablyarranged so that each of the ‘line’ of contacts are parallel to eachother. The separation between points in the array contacts can be equalto or less than 2 mm, preferably equal to or less than 1 mm.

While some of the embodiments discussed above reference laser processingto create point contacts through an intrinsic layer (e.g. intrinsiclayer 220) of a heterojunction, in some embodiments, laser processing isused to fire p⁺ and n⁺ point contacts through a dielectric coating (e.g.dielectric layer 295 in FIG. 4) over a rear heterojunction or reartunnel oxide structure. Further, another laser transfer process may thenbe used to deposit an interdigitated finger pattern of an appropriatemetal on top of the dielectric coating and over the appropriate pointcontacts using a narrow laser beam. In some embodiments, the laserprocessing conditions could be chosen so that the laser processingdisrupts the heterojunction or tunnel oxide layers in the vicinity ofthe base point contact, but not in the vicinity of the emitter pointcontacts. For example, in some embodiments, the Gaussian laser beam maybe small diameter beam, such as a beam that is ≦20 μm and ≧10 μm. Inanother embodiment, the laser beam may be a line shaped beam with onedimension that is 20 μm or less (e.g. width of the beam) and the otherdimension that is 80 μm or greater (e.g. length of the beam). Aninterdigitated seed layer (e.g. Ni) could be laser transferred on top ofthe heterojunction or tunnel oxide layers and on top of the n⁺ and p⁺point contacts. In another embodiment, a finger pattern of n+ and p+materials (e.g. Al and Sb) may be laser transferred under conditions tolay the transferred materials on top of the dielectric layers, and tothen laser fire the p⁺ and n⁺ point contacts through the dielectric andthe rear heterojunction or tunnel oxide layers. In yet anotherembodiment, the n⁺ and p⁺ point contacts may be laser transferred beforeforming the heterojunction or tunnel oxide junction on the wafer, thenlaser transferring a IBC pattern (e.g. Ni) on top of the dielectriclayer over the heterojunction or tunnel oxide layers. Subsequently, theNi may be laser fired into the point contacts. It should be noted thataccurate alignment may be desirable with this approach in order to laserfire the Ni into the point contacts.

In yet another embodiment, a laser processing system is provided thatutilizes either a narrow line-shaped laser beam or an array of smallGaussian laser beams (e.g. <20 μm or <10 μm), either of which can betemporally shaped, to either ablate, transfer a dopant, metal, or othermaterial; or laser-dope or laser fire localized p⁺ or n⁺ contacts. Thissystem uses the spatially shaped laser beams to transfer and laser-fire(or laser dope) both p⁺ and n⁺ dopants through an optional dielectriclayer over a heterojunction or tunnel oxide junction to form a low-cost,high-performance, interdigitated back-contact solar cell at lowtemperatures without the need for any vacuum processing equipment. Thelaser processing system may comprise a laser beam with a temporallyadjustable pulse that is optimized to produce high quality localizedemitters and base contacts. The transparent transfer substrate (e.g. athin glass plate) is held a fixed distance from the Si wafer (e.g. 5-50microns) and can be moved between regions containing layers of variousmaterials, such as metals (e.g Sb, Al), dopant materials (e.g. spin-onphosphorus or boron containing inks), or no coating so that the lasercan either transfer the metals or ablate a dielectric surface on the Siwafer. By designing the system with interchangeable optics, one couldlaser transfer and dope p⁺ and n⁺ point contacts and then switch to alow-power laser transfer of an interdigitated finger pattern that wouldlie on top of the dielectric layer over the heterojunction or tunneloxide layers. The laser beam can be scanned across the transparenttransfer substrate and the silicon wafer to form the desired contactpattern on the surface of the wafer.

Systems and methods for producing high-performance interdigitated backcontact (IBC) solar cells that are fabricated at low temperatures withlow manufacturing costs using a laser-transfer process are discussedherein.

Example

An n-type continuous Czochralski (CCz) silicon wafer with lightphosphorous doping (2.8 Ω-cm) was etched to remove saw damage in 20 wt %KOH at 80° C. for 12 minutes. After RCA cleaning, the wafer wassubjected to the following PECVD growth: (1) A layer of 10 nm ofintrinsic hydrogenated amorphous silicon followed by a layer of 72 nm ofsilicon nitride was grown on the front surface; (2) a layer of 10 nmintrinsic hydrogenated amorphous silicon followed by 10 nm of borondoped hydrogenated amorphous silicon (2% borane dopant feed), followedwith a final layer of 72 nm of silicon nitride was grown on the rearsurface.

The above sample was annealed at 180° C. for 30 minutes in air in a tubefurnace. An antimony donor plate comprising 50 Å of Ni followed by 2000Å of antimony on 700 μm thick borosilicate glass was prepared. Thesilicon sample was laser fired by placing the antimony donor plateantimony side down on the silicon and laser firing through the antimonydonor plate with a 100 ns laser pulse at 1064 nm wavelength. The laserpower and focus were chosen such that the laser fire mark on the samplewas about 200 μm long by approximately 10 μm wide. The firing patterncomprised laser fired features measuring 17 mm in the X direction and 50μm in the Y direction (e.g. see FIG. 7d ). The laser fired featurescontained individual laser line patterned spots separated by 500 micronin the X direction and 5 micron in the Y direction. These lines wereplaced on the sample at a 2 mm spacing in the Y direction.

Silicon nitride was then completely removed from that side of thesample, and a metal layer comprising 50 nm of Aluminum and 400 nm ofsilver was deposited by evaporation. Microposit s1813 positivephotoresist was applied to the sample and then exposed to create theisolation line (e.g. see 260 from FIG. 9d ). After development of thephotoresist, the isolation was etched with PAN etch (16:1:1:2H3PO4:HNO3:Acetic acid:Water) for 10 minutes to isolate the base andemitter contacts as defined in the structure of FIG. 2. The sample wasannealed for 30 minutes at 180 C in air.

The resulting device had an active area of 2.6 cm² and was tested for IVcharacteristics. The solar cell displayed an open circuit voltage of0.608V, a short circuit current of 29.3 mA/cm2, and an efficiency of11.1%.

Embodiments described herein are included to demonstrate particularaspects of the present disclosure. It should be appreciated by those ofskill in the art that the embodiments described herein merely representexemplary embodiments of the disclosure. Those of ordinary skill in theart should, in light of the present disclosure, appreciate that manychanges can be made in the specific embodiments described and stillobtain a like or similar result without departing from the spirit andscope of the present disclosure. From the foregoing description, one ofordinary skill in the art can easily ascertain the essentialcharacteristics of this disclosure, and without departing from thespirit and scope thereof, can make various changes and modifications toadapt the disclosure to various usages and conditions. The embodimentsdescribed hereinabove are meant to be illustrative only and should notbe taken as limiting of the scope of the disclosure.

What is claimed is:
 1. A method for forming interdigitated back contactsof a solar cell, the method comprising: forming a heterojunctionstructure or a tunnel junction structure on a rear surface of a mainsubstrate; maintaining a donor substrate at a distance from the mainsubstrate; forming a laser processed region of a base contact on therear surface, wherein the laser processed region of the base contact isformed by laser processing comprising a laser transfer process causingformation of a dopant on the main substrate via transfer of the dopantor a derivative thereof from the donor substrate and a laser-dopingprocess of the dopant deposited on the main substrate causing formationof the laser processed region; and depositing a metal layer in aninterdigitated finger pattern on the rear surface, wherein theinterdigitated finger pattern provides a first set of fingers of themetal layer in electrical communication with emitter regions and asecond set of fingers of the metal layer in electrical communicationwith the base contact, and the base contact is isolated from the emitterregions.
 2. The method of claim 1, where the laser processing for thelaser processed region of the base contact is a laser-firing orlaser-doping process that drives a dopant through the heterojunctionstructure or the tunnel junction structure into the main substrate. 3.The method of claim 1, wherein the laser transfer process causes thetransfer of the dopant onto base contact regions of the main substrate,and the laser-doping process causes the laser processed region to beformed by driving the dopant into and through the heterojunctionstructure or the tunnel junction structure to the main substrate.
 4. Themethod of claim 1 further comprising an additional laser processing stepto cause localized disruption to create an isolation gap between thebase contact and the emitter regions.
 5. The method of claim 1, whereina portion of the heterojunction structure or the tunnel junctionstructure is removed prior to forming the laser processed region.
 6. Themethod of claim 5, wherein the portion of the heterojunction structureor the tunnel junction structure removed is removed by laser ablation,or by delamination due to localized heating during the laser processing,or by masking and chemical etching.
 7. The method of claim 5, whereinthe portion of the heterojunction structure or the tunnel junctionstructure removed is from base contact regions of the main substrate. 8.The method of claim 5, wherein the portion of the heterojunctionstructure or the tunnel junction structure removed extends through theheterojunction structure or the tunnel junction structure and into aportion of the main substrate.
 9. The method of claim 5, wherein theportion of the heterojunction structure or the tunnel junction structureremoved is a region between the base contact and the emitter regions andprovides an isolation gap between the base contact and the emitterregions.
 10. The method of claim 1 further comprising the step of:applying an isolation layer on the heterojunction structure or thetunnel junction structure prior to depositing the metal, wherein theisolation layer is present between base contact regions and the emitterregions.
 11. The method of claim 1 further comprising the steps of:depositing an insulating layer on a portion of the metal layer in theinterdigitated finger pattern and into gaps in the metal layer betweenbase contact regions and the emitter regions; and depositing an extrametal on the metal layer and the insulating layer, wherein the extrametal is patterned to provide base contact regions of the extra metaland emitter contact regions of the extra metal, and a base contact widthof the base contact regions is approximately equal to an emitter contactwidth of the emitter contact regions.
 12. The method of claim 1 furthercomprising the step of: depositing a dielectric layer on theheterojunction structure or the tunnel junction structure prior to theforming the laser processed region of the base contact, wherein at leasta portion of the dielectric layer is absent from the emitter regions.13. The method of claim 1, where the emitter regions occupy 60% or moreof the rear area of the cell.
 14. The method of claim 1, wherein thelaser processing utilizes a spatially profiled laser beam or atemporally shaped laser beam.
 15. The method of claim 1, wherein thelaser processing utilizes one or an array of Gaussian beams where theGaussian beams are circular, and where each of the Gaussian beams has adiameter of 30 microns or less.
 16. The method of claim 1, wherein thelaser processing utilizes a laser beam that is narrow and line-shaped,and the width of the laser beam is 20 microns or less.
 17. The method ofclaim 1, wherein a laser beam is temporally shaped with an initialenergy density over a first time period to transfer the dopant materialfrom the donor substrate to the main substrate during the laser transferprocess, and the laser beam also disrupts the heterojunction layer orthe tunnel junction layer to cause delamination, the laser beam istransitioned to a second energy density that is lower that the initialenergy density for a second time period to locally melt theheterojunction layer or the tunnel junction layer and to allow thedopant to diffuse into the main substrate during the laser-dopingprocess, and the laser beam is transitioned to a third energy densitythat is lower than the second energy density for a third time period toanneal the localized laser-processed region.
 18. The method of claim 1,wherein the laser-doping process causes localized disruption of aportion of the heterojunction structure or the tunnel junction structureand simultaneously drives the dopant through the heterojunctionstructure or the tunnel junction structure into the main substrate.